feat(sqlite): initial stable release v0.9.0

Pure-Go CGO-free SQLite client with launcher lifecycle, write-mutex serialisation, health check, unit-of-work via context injection, and structured error mapping.

What's included:
- Executor / Tx / Client / Component interfaces using database/sql native types
- Tx.Commit() / Tx.Rollback() without ctx, matching the honest database/sql contract
- New(logger, cfg) constructor; database opened in OnInit
- Config struct with env-tag support; default Pragmas: WAL + 5s busy timeout + FK enforcement
- PRAGMA foreign_keys = ON enforced explicitly in OnInit
- writeMu sync.Mutex acquired by UnitOfWork.Do to serialise writes and prevent SQLITE_BUSY
- UnitOfWork via context injection; GetExecutor(ctx) returns active Tx or *sql.DB
- HandleError mapping SQLite extended error codes to xerrors codes (unique/primary-key → AlreadyExists, foreign-key → InvalidInput, ErrNoRows → NotFound)
- health.Checkable at LevelCritical; pure-Go modernc.org/sqlite driver (CGO_ENABLED=0 compatible)

Tested-via: todo-api POC integration
Reviewed-against: docs/adr/

docs/adr/ADR-002-write-mutex-busy-prevention.md

@@ -0,0 +1,53 @@
+# ADR-002: Write Mutex to Prevent SQLITE_BUSY Under Concurrent Load
+
+**Status:** Accepted
+**Date:** 2026-03-18
+
+## Context
+
+SQLite uses file-level locking. When multiple goroutines attempt write transactions
+concurrently, SQLite cannot acquire the write lock immediately and returns `SQLITE_BUSY`.
+Although the default Pragmas configure a 5-second busy timeout (`_timeout=5000`), this is a
+passive wait that still allows competing transactions to collide and fail under sustained
+concurrent write pressure.
+
+WAL mode (`_journal=WAL`) improves read concurrency but does not eliminate write contention:
+SQLite still allows only one writer at a time.
+
+## Decision
+
+The `sqliteComponent` holds a `writeMu sync.Mutex` field. `NewUnitOfWork` detects when its
+`Client` argument is the concrete `*sqliteComponent` type and extracts a pointer to that mutex.
+`unitOfWork.Do` acquires the mutex before beginning a transaction and releases it after
+commit or rollback:
+
+```go
+func (u *unitOfWork) Do(ctx context.Context, fn func(ctx context.Context) error) error {
+    if u.writeMu != nil {
+        u.writeMu.Lock()
+        defer u.writeMu.Unlock()
+    }
+    tx, err := u.client.Begin(ctx)
+    ...
+}
+```
+
+This serialises all write transactions at the application level, guaranteeing that only one
+writer reaches SQLite at a time and eliminating `SQLITE_BUSY` errors entirely.
+
+The mutex is only applied when using `NewUnitOfWork`. Callers who manage transactions manually
+via `Begin`/`Commit`/`Rollback` are not protected and must handle contention themselves.
+
+## Consequences
+
+**Positive:**
+- `SQLITE_BUSY` is eliminated for all write workloads going through `UnitOfWork`.
+- Behaviour is deterministic and testable (see `TestUnitOfWork_WriteMutex`).
+- Reads are unaffected; the mutex only wraps writes.
+
+**Negative:**
+- Write throughput is bounded to one goroutine at a time. This is acceptable for SQLite's
+  typical deployment profile (embedded, single-process, modest write rates).
+- The type assertion `client.(*sqliteComponent)` couples `NewUnitOfWork` to the concrete type.
+  When a mock or alternative `Client` is supplied, `writeMu` is `nil` and serialisation is
+  skipped silently. This is intentional for testing flexibility.